1. Field of the Invention
The present invention relates to a developing device having a two-component developer including magnetic particles and toner, and to an image forming apparatus comprising the same.
2. Description of the Related Art
Well known in the past are developing devices that supported a two-component developer comprising non-magnetic toner particles and magnetic carrier particles on the surface of a developer carrier and used two-component developer to developed electrostatic latent images formed corresponding to the image information on a photoconductive member that was the image carrier. This developing device transported and supplied developer, which formed a so-called magnetic brush on the surface of the developer carrier, to the vicinity of the surface of a photoconductive member that supported the electrostatic latent image, and by applying DC developing bias (causing AC component superimposition as necessary) to the developer carrier which faced the photoconductive member while maintaining a minute gap, developed and manifested the electrostatic latent image with toner particles from the developer carrier side to the photoconductive member side.
The developer carrier of the brush type developing devices that develop images by forming a magnetic brush in this way is commonly configured by a developing sleeve formed in a cylindrical shape, and a magnetic roller comprising multiple magnetic poles arranged in the interior of the developing sleeve. This magnetic roller is for the purpose of forming a magnetic field that makes spikes of developer stand on the surface of the developing sleeve. The spikes of developer are transported to the surface of the developing sleeve by the relative movement of the developing sleeve in relation to this magnetic roller. In the developing region, the developer on the developing sleeve spikes up along the lines of magnetic force generated from developer magnetic poles that the magnetic roller has. The developer that is spiked up and formed into a brush shape flexibly makes contact with the surface of the developer carrier in association with the movement of the surface of the developing sleeve, and supplies toner to the electrostatic latent image.
Moreover, there may be various types of developing sleeves that carry the developer, but the type generally used is the one in which the surface of the developing sleeve has been roughened. Making multiple grooves extending longitudinally on the surface, and such processes as sandblasting, etc. to contour the surface may be used as the processes to roughen the surface of the developing sleeve. In contrast with superior developer transport capacity, the former type that has grooves is prone to generate sleeve pitch concentration irregularities on the image because increases and decreases of the amount of developer carried are produced in the sleeve circumferential direction in conjunction with the presence or absence of the grooves. Meanwhile, in the blast finish developing sleeve, the type of abnormal image described above is not produced in association with groove pitch, and therefore the blast finish sleeves are preferable in terms of achieving high image quality in an image forming apparatus that outputs full color images.
With the recent color advances in electronic photographic systems, the demand for high image quality and high reproducibility has become heightened. Yellow, magenta and cyan colored toners are used for full color electronic photographic toner. Further, black toner is also used as necessary. It is desirable that the particles of toner have a small particle size in order to obtain high resolution and clear images. However, reducing particle size produces the side effect of a notable decrease in fluidity in association with degradation of the developer.
This kind of decrease in developer fluidity appears to be produced by the following factors. Specifically, the developer on the developer carrier is restricted by a developer restricting member, thus restricting the amount of developer to be transported to the development region, but when passing through this developer restricting member, the developer undergoes large mechanical stress. This mechanical stress is a factor in burying the external additive, which was applied to the exterior of the toner in order to provide fluidity, and in scraping off the resin adhering to the surface of the carrier.
Because the developer transport capacity is weaker compared to developing sleeves having grooves, blast finished developing sleeves have a greater reduction in the amount of developer carried in association with this reduction of developer fluidity. As a result, the development capacity is reduced and the image concentration decreases. Countermeasures to suppress this kind of reduction in development capacity include: (1) increasing the linear velocity of the developing sleeve more than the linear velocity of the photoconductive member; (2) raising the development potential; and (3) heightening the toner concentration of the developer and reducing the electrostatic charge of the developer. However, when using countermeasure (1) of increasing the linear velocity of the developing sleeve more than the linear velocity of the photoconductive member, the developer rubs and abrades the photoconductive member in the development region, and the carrier produces frictional electrostatic charge with a reverse polarity to the toner, thus manifesting the problem of so-called “carrier adhesion”, in which the carrier adheres to the photoconductive member. Moreover, when using countermeasure (2) of increasing the development potential, carrier with weak magnetic characteristics is developed on the photoconductive member, and the problem of carrier adhesion once again becomes manifest. In addition, the increase in the amount of charge passing through also raises the problem of shortening the working life of the photoconductive member. Moreover, when using countermeasure (3) of heightening the toner concentration of the developer and reducing the electrostatic charge of the developer, the problems of toner scattering and scum become manifest.
Anticipating a reduction in the amount of developer carried in association with the reduction of fluidity of the developer, the space between the developer restricting member and the developing sleeve may be pre-set wider, and the initial amount of developer carried may be set higher. However, simply setting the amount of developer carried higher will lead to supplying excessive developer to the development region, producing the so-call “developer retention” in which developer is retained between the photoconductive member and the developing sleeve. When this kind of developer retention is produced, developer drops from the ends of the developing sleeve. In addition, the developer retained between the photoconductive member and the developing sleeve receives stress between the photoconductive member and the developing sleeve, and developer adheres to the developing sleeve.
In addition, as a result of the increasing amount of developer supplied to the developing region, the length of the magnetic brush becomes longer, thereby lengthening the period of contact between the photoconductive member and the developer. Toner drift is prone to occur at the tip of the magnetic brush, wherein toner adhering to the surface of the carrier moves to the developing sleeve side by electrostatic force received from the non-latent image part during the period of facing the non-latent image part. Consequently, if the magnetic brush after undergoing toner drift rubs and abrades the back end of the latent image, the toner supply capacity decreases, and the so-called “scavenging phenomenon” occurs wherein the toner adhering to the back end of the latent image is electrostatically attracted and scratched away. Back end outlines and fine line reproducibility are reduced.
Specifically, recently increased linear velocity of the developing sleeve has been sought in conjunction with the development of high-speed image forming apparatuses, and all margin for scattering and sleeve adhesion, etc. is lost. Even more recently, the fixing unit has no oil coating function, and an oil-less color toner that contains releasing agent has also come on the market, but low boiling point releasing agent is prone to fuse to the surface of the developing sleeve, and from the perspective of guaranteeing the transport capacity of the developer, this is a disadvantage. In this way, in a color imaging for which image quality is emphasized, important technical issues in terms of supporting image quality over time include both stabilizing the amount of developer carried and handling high speeds.
For example, described in Japanese Patent Application Laid-open No. 2006-23783 (called Prior Art 1 hereinafter), is a technology in which the attenuation rate of the magnetic flux density in the normal direction of the developing sleeve surface of the main magnetic poles which cause the magnetic brush to spike up is 40% or more in the development region in order to prevent developer from adhering to blast-finished developing sleeves. The magnetic brush spike length can thereby be shortened, and a drop in back end outline and fine line reproducibility can be restricted when setting an initial high amount of amount developer carrier.
Moreover, described in Japanese Patent Application Laid-open No. 2005-62476 (called Prior Art 2 hereinafter), is a technology in which, an apparatus with a photoconductive member, a groove type developing sleeve, and a development gap G of 0.1 to 0.3 mm, the relationship ρ/G between the amount of developer ρ (mg/mm2) supplied to the development region and the development gap G is less than 2.5 (mg/mm3) in order to prevent “developer retention”.
In addition, described in Japanese Patent Application Laid-open No. 2005-37878 (called Prior Art 3 hereinafter), is a technology that fulfills the relationship between the layer thickness Tup of the developer layer prior to the developer passing through the restricting member and the gap Gd between the developer restricting member and developing sleeve is 7<(Tup/Gd)<20 in order to suppress degradation of the developer.
Nonetheless, the aforementioned Prior Art 1 cannot suppress “developer retention”, and cannot suppress developer scattering and developer adhesion. Moreover, if the fluidity of the developer decreases and the amount of developer carried declines, then the concern arises that sufficient spike length cannot be formed and the concentration decreases, etc.
Moreover, if the grooves of the aforementioned Prior Art 2 are used in a blast type developer sleeve, the image concentration will decrease due to a drop in the amount of developer carried based on a reduction of developer fluidity.
In addition, the aforementioned Prior Art 3 cannot restrict “developer retention”, and cannot suppress developer scattering and developer adhesion. In Prior Art 3, the period up to degradation of the developer can be extended, but when the developer degrades, the amount of developer carried decreases, reducing the image concentration.
In this way, no developing device in the past could address all the crucial aspects of developer retention, decreased developer fluidity and decreased amount of developer carried in order to guarantee high resolution, high grade images over a long period. Then, as a result of assiduous study, the inventors discovered the configuration of a developing device that could resolve all of the aforementioned issues. Specifically, by fulfilling the following conditions, developer retention, the decrease in developer fluidity, and the associated decreased amount of developer carried can be suppressed, and high grade images can be guaranteed over a long time.
(1) The amount of developer carried per unit area on the developer carrier in the developing region where toner on the developer carrier is moved to the image carrier side should be 30 [mg/cm2] or more and 60 [mg/cm2] or less.
(2) The toner weight mean particle diameter should be 4.5 [μm] or more and 8.0 [μm] or less, and the ratio [Dw/Dn] of the toner weight mean particle diameter (Dw) and the number mean particle diameter (Dn) should be 1.20 or less.
(3) The maximum height Rz of the surface roughness of the developer carrier should be 20 to 40 [μm], the mean space Sm of the roughness of the developer carrier surface should be 100 to 200 [μm], and the surface roughness of the developer carrier should have an irregular height and space roughness pattern.
(4) The value, which is obtained by dividing the gap DG between the developer carrier and a developer restricting member provided opposite to the developer carrier and restricting the amount of developer transported to the development region, by the developing gap PG between the image carrier and the developer carrier, should be 1.0 or more and 3.0 or less.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Application Laid-open No. 2003-177602, Japanese Patent Application Laid-open No. 2002-091053, and Japanese Patent Application Laid-open No. 2000-075541.